The present invention relates to a process for methanol reforming, in which the gas mixture to be reformed is passed through a catalyst-containing reaction compartment, and to an apparatus suitable for carrying out this process.
A known reforming process is used, in particular, for producing hydrogen gas. The methanol, for example together with water, is reacted in a hot-steam reformer to form hydrogen and carbon dioxide. The carbon dioxide, together with the hydrogen, in turn being in a reaction equilibrium with water and carbon monoxide (CO). Methanol conversion rate and carbon monoxide production are therefore factors which are connected to one. another via, among other things, the temperature-dependent reaction equilibria. That is, depending on the methanol conversion rate set by the process technology, a certain amount of carbon monoxide results when the currently known catalyst materials are used.
In many instances, the carbon monoxide is an undesirable by-product because of its toxicity for the environment and the catalyst materials used, and must frequently be removed in a complex manner. Whereas for an industrial large-scale process, under steady-state conditions, an optimum operating point can still be maintained comparatively simply, under non-steady-state conditions, particularly, which means, principally, throughputs varying with time of the gas mixture to be reformed and associated changes in the catalyst space velocity in the reformer, operating phases may occur in which a relatively large amount of carbon monoxide is formed. Non-steady-state conditions of this type exist, for example, in non-steady-state applications of hydrogen as an energy source, such as for peak current generation and in motor vehicle drive units, in which it is desired to have recourse to methanol as a liquid source of hydrogen in order not to require direct storage of hydrogen gas.
Because the methanol decomposition reaction proceeds more vigorously endothermically and the carbon monoxide formation from carbon dioxide proceeds more feebly endothermically, to minimize the CO formation, it has already been repeatedly proposed to establish a temperature gradient along the reaction path of the gas mixture in the reaction compartment. The temperature decreases from the inlet side to the outlet side of the reaction compartment.
Thus, in U.S. Pat. No. 4,865,624, a process and an apparatus for methanol reforming are described in which, by suitable heating circuits, an inlet-side half of a catalyst-containing reaction compartment through which the gas mixture to be reformed can flow is kept at a higher temperature in a co-current flow process, and a subsequent outlet-side half is kept at a lower temperature in a counter-current flow process. This is said to promote the methanol conversion in the intake stage and the conversion of carbon monoxide into carbon dioxide in the exit stage.
In a similar manner, DE 41 93 026 T1 describes the establishment of three temperature zones, situated one after the other, of approximately 300.degree. C., approximately 275.degree. C. and approximately 225.degree. C. along the reaction path in a methanol reformer. In JP 63-50302(A), a stepwise temperature decrease in the reaction compartment of a methanol reformer along the gas mixture flow path to decrease CO formation is likewise specified. In all of these conventional processes, the parameters of the reforming reaction are preset independently of the respective instantaneous throughput of gas mixture to be reformed.
An object of the present invention is to provide a process and an apparatus suitable for carrying out that process, by way of which methanol, even in the case of varying throughputs of gas mixture to be reformed (e.g., as a result of fluctuating hydrogen gas requirement) can be reformed so that the reformate leaving the reaction compartment contains relatively little carbon monoxide.
This problem has been solved by a process in which passing the gas mixture to be reformed through a catalyst-containing reaction compartment, and setting at least one of active length and active inlet cross-section of an intake-side reaction compartment section which is temperature controlled to achieve a high methanol conversion rate as a function of the respective throughput of gas mixture to be reformed. As a result, a residence period of the gas mixture remains essentially constant in the intake-side reaction compartment section.
An apparatus which utilizes the process of the present invention has a device for variable setting of the heating fluid volumetric flow rate in the heating fluid circuit, and is configured so as to actuatable as a function of throughput of gas mixture to be reformed. Thereby, at least one of active length and active inlet cross-section of an intake-side reaction compartment section which is temperature controlled to achieve a high methanol conversion rate as a function of the respective throughput of gas mixture to be reformed. As a result, a residence period of the gas mixture remains essentially constant in the intake-side reaction compartment section or a heating space which is in operative thermal contact with an intake-side reaction compartment section which is temperature-controlled for high methanol conversion.
The heating space in the present invention is delimited by a wall which can be slid in parallel to a direction of flow of the gas mixture to be reformed such that at least one of active length and active inlet cross-section of an intake-side reaction compartment section which is temperature controlled to achieve a high methanol conversion rate as a function of the respective throughput of gas mixture to be reformed. Consequently, a residence period of the gas mixture remains essentially constant in the intake-side reaction compartment section. Alternatively, a device optionally intake-side opens or blocks a variable number of the reactor tubes so that at least one of active length and active inlet cross-section of an intake-side reaction compartment section which is temperature controlled achieves a high methanol conversion rate as a function of the respective throughput of gas mixture to be reformed and a residence period of the gas mixture remains essentially constant in the intake-side reaction compartment section.
With regard to the process, the active length of the intake-side reaction compartment section, which is temperature-controlled to achieve a high methanol conversion rate, and/or its active cross-section is variable as a function of the particular throughput of gas mixture to be reformed such that the residence period of the gas mixture to be reformed remains essentially constant in the reaction compartment section temperature-controlled to achieve a high methanol conversion rate. Thereby, even under non-steady-state operating conditions, an optimum operating state with respect to minimum CO formation can be set in the methanol reformer.
Underlying the process of the present invention is the experimentally confirmed discovery that the methanol decomposition reaction, on one hand, and the CO formation reaction, on the other hand, proceed. at different speeds at different values of parameters such as reaction compartment temperature and residence period in the reaction compartment of the gas mixture to be reformed. Thus, for each combination of reaction compartment temperature and residence period, an operating state optimum exists having high methanol conversion and low CO formation.
The process of the present invention always keeps the operating point of the methanol reformation at this optimum operating state, as a function of the possibly fluctuating gas mixture throughput. Experimentally confirmed considerations of the reaction procedure indicate that, in the methanol reformation, the methanol is first reacted and carbon monoxide is only formed to an increasing extent in an exit-side part of the reactor path length no longer utilized for methanol conversion at low throughputs.
By matching the active length of the intake-side reaction compartment section, which is temperature-controlled to achieve a high methanol conversion rate, to the respective instantaneous gas mixture throughput, it is therefore always possible by the process of the present invention to avoid such a downstream reaction compartment section which is heated to a higher temperature and to which virtually no methanol passes any more and which causes Co formation to an increased extent. Additionally, or alternatively, to this length matching, the residence period of the gas mixture to be reformed can be kept essentially constant, independently of the respective gas mixture throughput, in the intake-side reaction compartment section which is temperature-controlled. A high methanol conversion rate is thereby achieved by the fact that the active intake-side reaction compartment cross-section is changed as a function of the respective gas mixture throughput.
According to another feature of the process according to the present invention, the length of the intake-side reaction compartment section active for the methanol conversion is matched by the fact that the volumetric flow rate of the heating fluid used for the temperature control of the reaction compartment is set appropriately, and the associated heating fluid circuit is operated in co-current flow. That is, the heating fluid flows in the area of thermal contact with the reaction compartment in parallel to the gas mixture to be converted. The temperature gradient along the reaction compartment can thus be adjusted, and the length of the reaction compartment section which is temperature-controlled can be adjusted to achieve a high methanol conversion rate.
In another embodiment of the process according to the present invention, the length of the intake-side reaction compartment section is matched by the length of the part of the heating fluid circuit in thermal contact with the reaction compartment being varied suitably.
According to another aspect of the process of the present invention, the intake-side reaction compartment active cross-section for high methanol conversion is changed by optionally individually releasing or blocking reaction part-compartments which are separate from each other and arranged in parallel.
With the apparatus according to the present invention, the volumetric flow rate of the heating fluid serving for the temperature control of the reforming reaction compartment can be advantageously altered in the associated heating fluid circuit via the device provided for this purpose, which enables the above-described process to be carried out.
The apparatus according to the present invention can have the heating fluid circuit for the temperature control of the reaction compartment which contains a temperature-control compartment in thermal contact with the reaction compartment. The temperature-control compartment is delimited by a wall which can be displaced in parallel to the direction of flow of the gas mixture to be reformed. By displacing the wall, the length of the thermal contact between temperature-control compartment and reaction compartment can therefore be set. Thus, for example, the process of the present invention can be carried out insofar as matching the length of the intake-side reaction compartment section by suitably varying the length of the part of the heating circuit in thermal contact with the reaction compartment.
The apparatus can also be configured with a tube-bundle reformer, in which a desired number of reactor tubes can be released or blocked to carry out, e.g., the process of changing the intake-side reactive compartment cross-section.